Ion exchange fluorocarbon resin membrane

ABSTRACT

An ion exchange fluorocarbon resin membrane having a storage modulus (JIS K-7244), when substituted with tributyl ammonium ions, at a temperature of not less than Tc represented by the following general equations (1-1) to (1-3) of 0.8×10 6  Pa or more, and a heat of crystal fusion at 270 to 350° C. of 1 J/g or less:
 
 Tc  (° C.)=55+ Tg  ( EW&lt; 750)  (1-1)
 
 Tc  (° C.)=0.444× EW− 278+ Tg  (750≦ EW&lt; 930)  (1-2)
 
 Tc  (° C.)=135+ Tg  ( EW≧ 930)  (1-3)
 
where Tg designates the peak temperature of loss tangent in the dynamic viscoelasticity measure-ment of the ion exchange fluorocarbon resin membrane (having SO 3 H as the ends of the side chains).

TECHNICAL FIELD

The present invention relates to an ion exchange fluorocarbon resinmembrane used as an electrolyte and a diaphragm of a solid polymer typeof fuel cell, in particular to an ion exchange fluorocarbon resinmembrane having excellent performance as an electrolyte and a diaphragm.

BACKGROUND ART

A fuel cell is a sort of electric generator that generates electricenergy by electrochemically oxidizing fuels such as hydrogen andmethanol, and has lately attracted attention as a clean energy source.Fuel cells are classified into a phosphoric acid type, a moltencarbonate type, a solid oxide type, a solid polyelectrolyte type or thelike according to the kind of the electrolyte to be used. Among thesethe solid polyelectrolyte type of fuel cell is expected to be widelyapplied as a power source for electric vehicles or the like because ofits low standard operating temperature, as low as 100° C. or below, andits high energy density.

The solid polyelectrolyte type fuel cell is basically composed of an ionexchange membrane and a pair of gas diffusion electrodes bonded to bothsides thereof. It generates electricity by supplying hydrogen to oneelectrode and oxygen to the other electrode while both electrodes areconnected to an external load circuit. More specifically, protons andelectrons are formed in the hydrogen side electrode. The protons migratethrough the ion exchange membrane to the oxygen side electrode, and thenreact with oxygen to form water, while the electrons flow through aconductor from the hydrogen side electrode and discharge electric energyin the external load circuit. They then arrive at the oxygen sideelectrode through another conductor, resulting in contributing to thecourse of the above-described water-forming reaction. Although arequired characteristic of the ion exchange membrane is high ionconductivity in the first place, high water content and high waterdispersibility in addition to the ion conductivity, are also importantrequired characteristics because protons are considered to be stabilizedby hydration of a water molecules when migrating through the ionexchange membrane. In addition, since the ion exchange membrane alsoplays the role of a barrier to prevent direct reaction of hydrogen andoxygen, low gas permeability is required. Furthermore, properties suchas chemical stability to resist a strong oxidation atmosphere during thefuel cell operation, and mechanical strength to meet the requirementsfor a thin membrane, are also necessary.

Ion exchange fluorocarbon resins are widely employed as a material forthe ion exchange membrane used in fuel cells of the solidpolyelectrolyte type, because of their high chemical stability. “Nafion”(registered trademark) manufactured by E.I. du Pont de Nemours andCompany having a perfluorocarbon as the main chains and sulfonic acidgroups at the end of side chains is widely used. Although such an ionexchange fluorocarbon resin has generally balanced properties as a solidpolyelectrolyte material, further improvements in the properties thereofhave been required with progress in the practical use of the fuel cells.

For example, although higher heat resistance has been increasinglydemanded, particularly in motor vehicle applications for preventingcatalyst poisoning and improving the cooling effect, it is said that theoperation of the present standard ion exchange fluorocarbon resinmembrane at 90° C. or above is difficult. Specifically, theabove-described higher heat resistance requires improvement of the heatresistance of ion exchange fluorocarbon resin membranes to 100° C. orabove, preferably 120° C. or above.

As means to improve the heat resistance of ion exchange fluorocarbonresin membranes, prior art techniques using the addition of reinforcingagents or block copolymerization, such as block copolymerization withPTFE (JP-A-11-329062), the addition of PTFE fibrils (JP-A-60-149631) orinorganic particles (JP-A-6-111827), as well as the formation of SiO₂networks by the sol-gel method (K. T. Adjemianetal, 2000 Fuel CellSeminar, pp. 164–166), are known. According to these prior arttechniques, although heat resistance was improved to some extent by theaddition of reinforcing agents in several percent by weight or blockcopolymerization, the lowering of ionic conductivity in exchange forincrased heat resistance became a problem because of the lowering of anapparent exchange capacity. On the other hand, in addition to theaddition of reinforcing agents, a prior art technique whereincross-linking functional groups are copolymerized with the precursor ofan ion exchange fluorocarbon resin membrane (JP-A-7-508779) are alsoknown. According to such a technique, although it is considered thatheat resistance can be compatible with ionic conductivity by properlydesigning cross-linking, there is a problem that cross-linking leads toincrease in costs, and in certain cases, the cross-linking reactiontakes a long time. As described above, prior art techniques related tothe improvement of heat resistance have essential problems, and have notbecome industrially useful techniques for ion exchange membranes forfuel cells.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a exchange fluorocarbonresin membrane superior in heat resistance.

An ion exchange fluorocarbon resin membrane can be manufactured forexample by melt-forming a precursor of the ion exchange fluorocarbonresin into the form of a membrane, and then hydrolyzing the membrane.

The present inventors paid attention to the storage modulus of theprecursor of the ion exchange fluorocarbon resin and found afterextensively repeated studies that the ion exchange fluorocarbon resinmembrane having a specific storage modulus manifested non-liquidity andmelt strength as a cross-linked membrane at a high temperature of theflow-starting temperature or above without treatment such ascross-linking or the modification of molecular structures, and thusaccomplished the present invention.

Namely, the present invention is as follows:

-   (1) An ion exchange fluorocarbon resin membrane having a storage    modulus (JIS K-7244), when substituted with tributyl ammonium ions,    at a temperature of not less than Tc represented by the following    general equations (1-1) to (1-3) of 0.8×10⁶ Pa or more, and a heat    of crystal fusion at 270 to 350° C. of 1 J/g or less:    Tc(° C.)=55+Tg(EW<750)  (1-1)    Tc(° C.)=0.444×EW−278+Tg(750≦EW<930)  (1-2)    Tc(° C.)=135+Tg(EW≧930)  (1-3)    where Tg designates the peak temperature of loss tangent in the    dynamic viscoelasticity measurement of the ion exchange fluorocarbon    resin membrane (having SO₃H as the ends of the side chains).-   (2) An ion exchange fluorocarbon resin membrane having a storage    modulus (JIS K-7244) of 1.0×10⁶ Pa or more at 200° C., a heat of    crystal fusion at 270 to 350° C. of 1 J/g or less, an equivalent    weight of 950 or less, and a Tg of 135° C. or above.-   (3) An ion exchange fluorocarbon resin membrane having a storage    modulus (JIS K-7244), when substituted with tributyl ammonium ions,    at a temperature Tc represented by the above-described general    equations (1) or above of 0.8×10⁶ Pa or more, and a molecular weight    of a fluorinated olefin chain in a polymer of 3,000 or less.-   (4) The ion exchange fluorocarbon resin membrane according to any    of (1) to (3), wherein a ratio of the storage modulus at 150° C. to    the storage modulus at 200° C. is 0.4 or more.-   (5) The ion exchange fluorocarbon resin membrane according to any    of (1) to (4), wherein a ratio of the storage modulus at Tc−50° C.    to the storage modulus at Tc is 0.4 or more.-   (6) The ion exchange fluorocarbon resin membrane according to any    of (1) to (5), wherein a high-temperature break strength at 220° C.    is 2 kg/cm² or more.-   (7) The ion exchange fluorocarbon resin membrane according to any    of (1) to (6), wherein the ion exchange fluorocarbon resin membrane    is not cross-linked.-   (8) A method of manufacturing an ion exchange fluorocarbon resin    membrane by forming a film of a precursor of the ion exchange    fluorocarbon resin, and hydrolyzing the same, wherein the precursor    of the ion exchange fluorocarbon resin has a heat of crystal fusion    at 270 to 350° C. of 1 J/g or less, and a melt index of 1 or less.-   (9) The method according to (8), wherein the precursor of the ion    exchange fluorocarbon resin has a melt index (JIS K-7210) of 0.5 or    less.-   (10) The method according to (8), wherein the precursor of the ion    exchange fluorocarbon resin has a melt index (JIS K-7210) of 0.4 or    less.-   (11) The method according to (8), wherein the precursor of the ion    exchange fluorocarbon resin has a melt index (JIS K-7210) of 0.1 or    less.-   (12) The method according to any of (8) to (11), wherein stretching    is performed in at least one direction after film formation.-   (13) The method according to (12), wherein stretching is performed    prior to hydrolysis, and the hydrolysis is performed while    maintaining orientation.-   (14) The method according to (12) or (13), wherein the temperature    for the stretching is 70° C. or above and below 300° C.-   (15) A membrane electrode composite comprising an ion exchange    fluorocarbon resin membrane according to any of (1) to (7).-   (16) A solid polyelectrolyte type fuel cell comprising an ion    exchange fluorocarbon resin membrane according to any of (1) to (7).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the relationship between the temperatures andstorage moduli on the film having SO₂F as the ends of the side chains inExamples 8, 11, 12, and 16 to 18.

FIG. 2 is a graph showing the relationship between the temperatures andstorage moduli on the film having SO₃H as the ends of the side chains inExamples 8, 10 to 12, and 15.

FIG. 3 is a graph showing the relationship between the temperatures andstorage moduli on the film having SO₃NH(Bu)₃ as the ends of the sidechains in Examples 8, 10 to 12, and 15.

BEST MODE FOR CARRYING OUT THE INVENTION

Although the precursor of the ion exchange fluorocarbon resin of thepresent invention has normally a rubber-like appearance, it isconsidered that a slight quantity of crystal components are present,because the precursor contains fluorinated vinyl compounds such as TFEas copolymerizing components. For example, when the dynamicviscoelasticity of the precursor of the ion exchange fluorocarbon resinhaving a standard melt index is measured at 35 Hz, a sharp lowering ofthe storage modulus is generally observed between 150 and 250° C., whichis considered to be a melt flow accompanying the melting of theabove-described crystal components. The feature of the ion exchangefluorocarbon resin membrane of the present invention is to exhibitnon-fluidity in a high-temperature range wherein a conventional ionexchange fluorocarbon resin membrane exhibits melt flow, and to manifesta certain mechanical strength (melt strength).

It has been well known that the phenomenon wherein a polymer manifestsmechanical strength without flowing at the melting temperature of thecrystal component or above occurs, for example, in ultra-high molecularweight polyethylene having a weight average molecular weight of1,000,000 or more. It is considered that such a phenomenon is inherentin ultra-high molecular weight polymers, and occurs because theentanglement of molecular chains is extremely difficult to disentangle,acting as a sort of cross-linked point. Although the relationshipbetween “molecular weight between entangling points” Me and “storagemodulus” E′ is represented by Me=ρRT/E′ (where ρ is density, R is gasconstant, and T is absolute temperature), the storage modulus, which isa requirement of the present invention, can be considered from theabove-described relational expression to represent the molecular weightbetween entangling points of molecular chains in the molten state. Sucha theory is described in detail, for example, in “Lecture-Rheology”,(The Society of Rheology, Japan, Kobunshi Kankokai, 1992). The presentinvention first clarified the critical conditions to make an ionexchange fluorocarbon resin membrane manifest the above-describednon-fluidity using the storage modulus as a main index, whereby the heatresistance of ion exchange fluorocarbon resin membranes could beimproved without relying on treatments such as cross-linking.Furthermore, the present invention enabled the industrial utilization ofthe heat-resistant membrane by disclosing the specific method forforming ultra-low MI polymers, which had no practical forming means inthe prior art.

(Material Polymers)

The precursor of an ion exchange fluorocarbon resin used in the presentinvention includes at least a binary copolymer of a fluorinated vinylcompound represented by the general formulaCF₂═CF—(OCF₂CFL)_(n)—O—(CF₂)_(m)—W and a fluorinated olefin representedby the general formula CF₂═CFZ, where, L is a F atom or a perfluoroalkylgroup having 1 to 3 carbon atoms, n is an integer of 0 to 3, m is aninteger of 1 to 3, and Z is H, Cl, F or a perflucroalkyl group having 1to 3 carbon atoms. Further, W is a functional group convertible to CO₂Hor SO₃H by hydrolysis, and as such a functional group, SO₂F, SO₂Cl,SO₂Br, COF, COCl, COBr, CO₂CH₃ or CO₂C₂H₅ are typically preferably used,and n is preferably 0 or 1.

Such an ion exchange fluorocarbon resin precursor can be synthesized byconventionally known means. For example, known methods include: a methodwherein the above fluorinated vinyl compound is dissolved usingpolymerizing solvents such as chlorofluorocarbons, then allow them toreact and polymerize with the fluorinated olefin gas (solutionpolymerization); a method of polymerization using the fluorinated vinylcompound itself as the polymerizing solvent without using a solvent suchas chlorofluorocarbons (bulk polymerization); a method wherein thefluorinated vinyl compound and a fluorinated olefin gas are charged intoan aqueous solution of an emulsifier to allow them to react andpolymerize (emulsion polymerization); a method wherein the fluorinatedvinyl compound and a fluorinated olefin gas are charged into an aqueoussolution of an emulsifier such as a surfactant and alcohol to emulsifythem, and allow them to react and polymerize (mini-emulsionpolymerization, micro-emulsion polymerization); and further a methodwherein the fluorinated vinyl compound and a fluorinated olefin gas arecharged into an aqueous solution of a suspension stabilizer to allowthem to react and polymerize (suspension polymerization). In the presentinvention, any of these polymerization methods can be used. As thefluorine-containing hydrocarbon used in solution polymerization, thecompounds generally referred to as chlorofluorocarbons such astrichlorotrifluoroethane and 1,1,1,2,3,4,4,5,5,5-decafluoropentane canbe preferably used.

(Equivalent Weight)

Although the equivalent weight (EW) of a the resin of the ion exchangefluorocarbon membrane of the present invention is not particularlylimited, it is preferably 400 to 1,400, more preferably 600 to 1,200,and further preferably 700 to 1,000. Although a higher equivalent weightenhances mechanical strength of even a non-oriented membranes, it lowersionic conductivity due to the lowering of the density of ion exchangegroups as well. An excessively low equivalent weight is also notpreferable because the strength is reduced.

(Melt Index)

The melt formability of the precursor of an ion exchange fluorocarbonresin is generally evaluated with the melt viscosity index known as meltindex (MI). The melt index (JIS K-7210) used herein is the weight of aresin extruded from a specific orifice when a load of 2.16 kg is appliedat 270° C. measured in grams per 10 minutes. Industry of ion exchangefluorocarbon resin membranes, precursors of ion exchange fluorocarbonresins having an MI of 10 to 50, which have good fluidity and excellentmelt formability, are typically used.

Although the melt index (MI) (JIS K-7210) of the precursor of an ionexchange fluorocarbon resin of the present invention is not particularlylimited, it is 10 or less, preferably 5 or less, more preferably 1 orless, especially preferably 0.5 or less, further preferably 0.4 or less,and extremely preferably 0.1 or less.

In the same EW, MI can be considered to be an index related to themolecular weight. However, when the EW is large (e.g., 1,000 or more),since the PTFE component is formed during polymerization, even duringrandom copolymerization, the PTFE component acts as a sort of filler,the precursor may have a low MI regardless of insufficient entanglingbetween molecules. In such a case, it is preferable to select a lower MIthan in the case of an EW of less than 1,000. For example, although themelt index (JIS K-7210) of the precursor of the ion exchangefluorocarbon resin membrane of the present invention when EW is 1,000 ormore is not particularly limited, it is generally 1 or less, preferably0.5 or less, more preferably 0.4 or less, especially preferably 0.1 orless, further preferably 0.06 or less, and extremely preferably 0.03 orless.

(Membrane Thickness)

The thickness of an ion exchange fluorocarbon resin membrane of thepresent invention is 1 to 500 μm, preferably 5 to 100 μm and morepreferably 10 to 50 μm. A membrane thickness below 1 μm tends to causethe above-described difficulty due to a diffusion of hydrogen or oxygen,along with difficulties such as damage of the membrane by a pressuredifference and strain during handling of the fuel cell duringmanufacturing or in operation. On the other hand, a membrane thicknessabove 500 μm may have an insufficient performance as an ion exchangemembrane because the membrane typically has a low ion permeability.

(Storage Modulus)

Heat resistance in the present invention is expressed through. thestorage modulus at Tc. The Tc is an index of the fluidizing temperatureof the polymer in the present invention, and can be considered to be thelower limit temperature at which various interactions bymicro-crystallites and the ends of side chains are eliminated, andwherein the viscoelasticity is controlled only by the entangling betweenmolecular chains. In the present invention, the heat resistance of anion exchange fluorocarbon resin membrane when used in a fuel cell isdetermined by the value of the storage modulus at Tc. Although thefluidizing temperature of the ion exchange fluorocarbon resin membraneis basically estimated to correspond to the melting temperature ofmicro-crystallites, and therefore shift toward the lower temperatureside with the lowering of EW, in the ion exchange fluorocarbon resinmembrane (having SO₃H as the ends of the side chains) used in a fuelcell, ideal behaviors related to the fluidizing temperature areconcealed by the following two factors, and as a result, it is clearthat the fluidizing temperature may not be suitable as the index of heatresistance.

The first factor which conceals ideal behaviors is sulfoniccross-linking of SO₃H ends with one another. Cross-linking is formed byreleasing one molecule of water from two molecules from SO₃H ends. Sincenew cross-linked structures are formed one after the other during themeasurement of the temperature dependence of storage modulus (duringtemperature elevation), even a low-molecular-weight polymer, which hasno entangling between molecular chains, exhibits a high storage modulusat Tc. Since such sulfonic cross-linking is significantly formed at orabove 220° C., a unique behavior is often observed wherein the storagemodulus once lowered by the fluidizing temperature at, for example,around 200° C. rises again to 220° C. or above.

In order to prevent the formation of sulfonic cross-linking, althoughthe protection of SO₃H ends by salt substituting is effective, thesecond factor related thereto is the reversion phenomenon of Tg and Tc.Here, Tg is the peak temperature of loss tangent in the measurement ofdynamic viscoelasticity of an ion exchange fluorocarbon resin membrane(having SO₃H as the ends of the side chains). For example, in an ionexchange fluorocarbon resin membrane having a Tg of 120° C., an EW of950 and an MI of 20, since the difference between the vicinity of thefluidizing temperature (200° C.) and Tg (120° C.) in the measurement ofdynamic viscoelasticity is as large as 80° C., fluidizing can be clearlyobserved without being affected by Tg. Whereas in an ion exchangefluorocarbon resin membrane substituted by potassium ions (having SO₃Kas the ends of the side chains), although sulfonic cross linking can beprevented, fluidizing in the vicinity of Tc cannot be observed becauseof the presence of Tg in the vicinity of 270° C., which is higher thanTc. In other words, no membranes are fluidized up to the vicinity of270° C.

The present inventors examined Tg for various salt-substituted ionexchange fluorocarbon resin membranes, and found that the Tg of themembrane wherein protons (hydrogen cations) in SO₃H as the ends of theside chains are substituted by alkyl ammonium ions, in particularsubstituted by tributyl ammonium ions, is significantly lower than theTg of the film wherein protons in SO₃H as the ends of the side chainsare substituted by alkali metal ions such as potassium ions or alkaliearth metal ions (equal to or lower than the Tg of membranes having SO₃Has the ends of the side chains). Fluidizing in the vicinity of Tc can beapparently observed.

Specifically, the storage modulus of the ion exchange fluorocarbon resinmembrane according to the present invention is measured with the ends ofthe side chains substituted with tributyl ammonium ions, that is, theends of the side chains are SO₃NH(Bu)₃.

The Tc of an ion exchange fluorocarbon resin membrane substituted bytributyl ammonium ions is defined by the following equations (1-1) to(1-3). For example, the Tc of the ion exchange fluorocarbon resinmembrane having a Tg of 120° C. and an EW of 800 is calculated to be197° C.; and the Tc of the ion exchange fluorocarbon resin membranehaving an EW of 1025 is calculated to be 255° C.Tc(° C.)=55+Tg(EW<750)  (1-1)Tc(° C.)=0.444×EW−278+Tg(750≦EW<930)  (1-2)Tc(° C.)=135+Tg(EW≧930)  (1-3)where Tg designates the peak temperature of loss tangent in the dynamicviscoelasticity measurement of the ion exchange fluorocarbon resinmembrane (having end of SO₃H as the ends of the side chain).

The Tc equation of the present invention is composed of a flat region atlow temperatures, a gradient region, and a flat region at hightemperatures. For example, when the equation of the gradient region (Tc(° C.)=0.444×EW−278+Tg) is applied to the ion exchange fluorocarbonresin membrane (having SO₃NH(Bu)₃ as the ends of the side chains) havinga Tg of 150° C. and an EW of 750, the Tc is calculated to be 205° C.,and since the difference between Tg and Tc at the lower EW becomes 55°C. or less, the Tc that has been considered to be the flow of the mainchain skeleton is strongly affected by Tg, which is the flow of the endof side chains (hereafter referred to as crossover). Specifically,although the Tc simply lowers depending on EW, since Tg is substantiallyconstant regardless of EW, the Tc as the fluidizing temperature has thelower limit derived from Tg. The manifestation of the flat region at lowtemperatures has such reasons. On the other hand, although the reasonfor the manifestation of the flat region at high temperatures has notbeen known, it is presumed to probably be because the effect of the endsof side chains are significantly weakened at an EW of 930 or more unlikethe flat region at low temperatures, and the action ofmicro-crystallites, not EW, becomes the main reason. The feature of theTc equation of the present invention is that the Tc equation isapplicable to any ion exchange fluorocarbon resin membrane described theabove-described paragraph of “Material polymers”, and the Tc can beuniquely determined from the Tg and EW of the ion exchange fluorocarbonresin membrane.

On the other hand, it has been known that the fluidizing temperature ofan acid-type ion exchange fluorocarbon resin membrane is 10 to 100° C.lower than the fluidizing temperature of an ion exchange fluorocarbonresin membrane substituted with tributyl ammonium ions, and in addition,when the above-described ion exchange fluorocarbon resin membrane hassufficiently low Tg and/or EW, the synergic effect may lower thefluidizing temperature by as much as 200° C. In this case, fluidizingcan be observed even in the acid-type ion exchange fluorocarbon resinmembrane without being affected by sulfonic cross-linking (which becomessignificant at 220° C. or above). Claim 2 of the present inventionaddresses an acid-type ion exchange fluorocarbon resin membrane (havingSO₃H as the ends of the side chains) characterized in that Tg is below135° C. and EW is less than 950, as the above-described ion exchangefluorocarbon resin membrane that does not necessarily requiresubstitution with tributyl ammonium ions.

The storage modulus (JIS K-7244) of such an acid-type ion exchangefluorocarbon resin membrane (having SO₃H the ends of the side chains) at200° C. is 0.8×10⁶ Pa or more, preferably 1.0×10⁶ Pa or more, morepreferably 1.5×10⁶ Pa or more, more preferably 1.8×10⁶ Pa or more,further preferably 2.0×10⁶ Pa or more, further more preferably 3.0×10⁶Pa or more, and especially more preferably 4.0×10⁶ Pa or more.

The storage modulus (JIS K-7244) of such an acid-type ion exchangefluorocarbon resin membrane of the present invention (having SO₃NH(Bu)₃as the ends of the side chains) at Tc is 0.8×10⁶ Pa or more, preferably1.0×10⁶ Pa or more, more preferably 1.5×10⁶ Pa or more, more preferably1.8×10⁶ Pa or more, further preferably 2.0×10⁶ Pa or more, further morepreferably 3.0×10⁶ Pa or more, and especially more preferably 4.0×10⁶ Paor more.

The fluidity at a temperature of the fluidizing temperature or above canbe expressed as the ratio of storage moduli before to after melting ofcrystal components. For example, in the case wherein the ends of theside chains are SO₃H, the ratio of the storage modulus at 150° C. to thestorage modulus at 200° C. is 0.4 or more, preferably 0.5 or more, morepreferably 0.6 or more, and further more preferably 0.7 or more. Whereaswhen the ends of the side chains are SO₃NH(Bu)₃, the ratio of thestorage modulus at 150° C. to the storage modulus at 200° C. is 0.4 ormore, preferably 0.5 or more, more preferably 0.6 or more, and furthermore preferably 0.7 or more. Furthermore, when the ends of the sidechains are SO₃NH(Bu)₃ the ratio of the storage modulus at Tc−50° C. tothe storage modulus at Tc is 0.4 or more, preferably 0.5 or more, morepreferably 0.6 or more, and further more preferably 0.7 or more.

(Heat of Crystal Fusion at 270 to 350° C.)

As described in the paragraph entitled “Melt index”, since the mixing ofa PTFE component not only causes the storage modulus to be differentfrom the object of the present invention, but also PTFE mixed withoutproper control brings about discontinuous change in mechanicalproperties in the boundary. The mixing of a PTFE component may easilycause the formation of pinholes due to stress concentration. Theexamples wherein PTFE is mixed without proper control include the caseof forming PTFE chains by block copolymerization, and the case ofintentionally forming PTFE particles during polymerization even inrandom copolymerization.

When the ion exchange fluorocarbon resin membrane is measured with DSC,a broad peak (half width: 50 to 100° C.) appears centered at 200° C. inmicro-crystallites corresponding to the fluidizing temperature, whereasin the PTFE component considered to be unsuitable for the presentinvention, a composite peak of a considerably sharper peak (half width:10 to 20° C.) and a broad peak can be observed between 270 and 350° C.because of large chain length. In the present invention, the heat ofcrystal fusion at 270 to 350° C. is 1 J/g or less, preferably 0.6 J/g orless, more preferably 0.4 J/g or less, further preferably 0.2 J/g orless, further more preferably 0.1 J/g or less, and most preferably 0.05J/g or less.

(Equivalent Puncture Strength)

Although the equivalent puncture strength (a converted value per 25 μmof a puncture strength in the dry state) of an ion exchange fluorocarbonresin membrane of the present invention is not particularly limited, itis preferably 250 g or more, more preferably 300 g or more, and furtherpreferably 350 g or more. An equivalent puncture strength less than 250g leads to insufficient mechanical strength due to thinning of themembrane and may not be preferable because the thickening of themembrane is required. Although the upper limit of equivalent puncturestrength is not particularly limited in the present invention, amembrane with a strength of 3,000 g or more is generally presumed tohave a low water content and thus insufficient performance as an ionexchange membrane.

(High-temperature Breaking Strength)

Although the high-temperature breaking strength (breaking strengthduring tensile test) of the ion exchange fluorocarbon resin membrane ofthe present invention at 200° C. is not particularly limited, it ispreferably 2 kg/cm² or more, more preferably 3 kg/cm² or more, furtherpreferably 4 kg/cm² or more, and most preferably 5 kg/cm² or more. Ifthe high-temperature breaking strength is less than 2 kg/cm², the ionexchange membrane may not exhibit sufficient performance in the initialproperties and the long-term durability in the operation of the fuelcell at a temperature of 100° C. or above. Although the high-temperaturebreaking strength of the ion exchange fluorocarbon resin membrane of thepresent invention at 220° C. is not particularly limited, it ispreferably 1 kg/cm² or more, more preferably 2 kg/cm² or more, furtherpreferably 3 kg/cm² or more, further more preferably 4 kg/cm² or more,and most preferably 5 kg/cm² or more. Furthermore, although thehigh-temperature breaking strength of the ion exchange fluorocarbonresin membrane of the present invention at Tc is not particularlylimited, it is preferably 2 kg/cm² or more, more preferably 3 kg/cm² ormore, and further preferably 4 kg/cm² or more.

In addition, although the tensile strength at normal temperature can beimproved by molecular orientation such as stretching, since many crystalcomponents melt at high temperatures as described above, much of suchmolecular orientation is relaxed. In other words, the characteristics ofthe polymer itself from which the contribution of molecular orientationis excluded, especially the entangling between molecules are stronglyreflected in the high-temperature breaking strength. It has been knownthat the time-temperature conversion rule can be generally applied topolymers in an amorphous state (including crystalline polymers in amolten state). According to the time-temperature conversion rule, theresults of measurement at a high temperature (e.g., 200° C.) in a shorttime (e.g., 35 Hz) are considered to reflect the behavior at lowtemperatures (e.g., 100 to 120° C.) in a long time (e.g., several hoursor more). In other words, the temperature at which the high-temperaturebreak strength is measured in the present invention is established onthe basis of the above-described point of view, combined with the objectto conduct the evaluation of durability for a long time at lowtemperatures in a simpler and easier way.

Next, a method of manufacturing the ion exchange fluorocarbon resinmembrane of the present invention will be described.

(Preferable Embodiment of a Manufacturing Method)

An ion exchange fluorocarbon resin membrane of the present invention isprepared through 1) a membrane-formation step and 2) a hydrolysis step.

(Membrane-formation Step)

As a method for forming a membrane from the precursor of an ion exchangefluorocarbon resin, any commonly known forming method can be suitablyused, including powder press methods, melt forming methods (T-diemethods, inflation methods, calendaring methods or the like) and castingmethods. The casting methods includes methods wherein an ion exchangefluorocarbon resin is dispersed in a suitable medium, or methodsincluding forming a sheet-like film from a polymerization reactionliquid itself, and then removing the dispersion medium. The resintemperature in melt forming using T-die methods is preferably 100 to300° C., and more preferably 200 to 280° C. The resin temperature inmelt forming by inflation methods is preferably 100 to 300° C., and morepreferably 160 to 240° C. Sheets that are melt formed by these methodsis cooled to the melting temperature or below using a chill roll or thelike.

Since the precursor of an ion exchange fluorocarbon resin according tothe present invention has a low melt index, it may be difficult toachieve the desired thickness of 1 to 500 μm by ordinary melt forming.In such a case, the desired thickness can be achieved by adding aplasticizer or the like to lower the viscosity thereof on melt forming,or by melt-forming a sheet thicker than desired, which sheet ismechanically thinned using various forming methods.

Although any plasticizer can be used as the plasticizer in the formermethod as long as it has the affinity with the precursor of the ionexchange fluorocarbon resin and can swell it, carbon dioxide,fluorine-containing hydrocarbons generally known as chlorofluorocarbons,or fluorine oil can be preferably used. Since these plasticizers havevarious boiling points, when a highly volatile plasticizer is used, atreatment such as compressing or chilling is preferable to prevent theevaporation of the plasticizer before, during or after forming.

The examples of the latter forming method include rolling using arolling machine, longitudinal uniaxial stretching using a rollstretching machine, transversal uniaxial stretching using a tenter,sequential biaxial stretching using a tenter and a longitudinal rollingmachine, and simultaneous biaxial stretching using a simultaneousbiaxial tenter. For example, if a precursor having an MI of about 0.1 isused, the formation of the sheet having a thickness of about 100 μmusing an ordinary T-die forming method is not unduly difficult if theT-die is specially designed. A precursor membrane having a thickness of25 μm can be formed after forming a precursor sheet, or furtherhydrolyzing to form an ion exchange fluorocarbon resin membrane, andperforming 2×2 stretching.

Although a preferable temperature range for stretching a precursor sheetis not particularly limited, stretching at a high temperature ispreferable, because the precursor sheet of the present invention has arelatively high molecular weight and strong entangling between moleculescompared to prior art. The lower limit of the temperature is preferablyroom temperature or above, more preferably 70° C. or above, furtherpreferably 90° C. or above, further more preferably 110° C. or above,and most preferably 130° C. or above. It is preferable for avoiding thedecomposition of the polymer that the stretching temperature does notexceed 300° C. After stretching the precursor sheet, it is preferable toperform hydrolysis in the state wherein the orientation is maintained toprevent relaxation of the orientation.

Similarly, although a preferable temperature range for stretching an ionexchange fluorocarbon resin membrane is not particularly limited, thelower limit of the temperature is preferably room temperature or above,more preferably 120° C. or above, further preferably 140° C. or above,and further more preferably 160° C. or above. After stretching the ionexchange fluorocarbon resin membrane, acid washing is preferable.

In any case, the molecules of the precursor of an ion exchangefluorocarbon resin membrane can be oriented two-dimensionally during theprocess of thinning. In this case, the mechanical strength is improved.On the other hand, although the precursor of an ion exchangefluorocarbon resin contains crystal components, it is essentially arubber-like polymer and simple molecular orientation may causeundesirable dimensional changes or strength reduction by the relaxationof orientation due to aging. If such problems are significant, it ispreferable to not impart molecular orientation in two-dimensionaldirections in the above-described thinning process. Methods for thinningwithout imparting two-dimensional molecular orientation include, forexample, temperature elevation in the above-described thinning process,or the use of a viscosity-lowering agents such as plasticizers. Amongthem, when the viscosity of the precursor sheet is lowered using ahighly volatile plasticizer, thinning can be performed without fear ofthe sublimation of the plasticizer from the precursor sheet, forexample, by installing the thinning machine, such as a rolling machine,in a bath filled with the plasticizer. On the other hand, whentwo-dimensional molecular orientation is to be maintained, the molecularorientation is fixed by performing hydrolysis while maintainingmolecular orientation during membrane forming. Dimensional change may beprevented, and at the same time, a high mechanical strength may beachieved.

(Hydrolysis Step)

As a method for hydrolysis, any commonly known methods may be used suchas the method described in Japanese Patent No. 2753731, wherein theprecursor of an ion exchange group of an oriented membrane is convertedto a metal-salt-type ion exchange group using a solution of alkalihydroxide, followed by converting to an acid type (SO₃H or COOH) ionexchange group using an acid such as sulfonic acid and hydrochloricacid. These conversions are known to those skilled in the art, anddescribed in Examples of the present invention. In the presentinvention, stretching or heat treatment may be performed during thehydrolysis step.

(Manufacturing Method for Membrane/electrode Assembly)

Next, a method for manufacturing a membrane/electrode assembly (MEA)will be described. MEA is manufactured by bonding electrodes to an ionexchange fluorocarbon resin membrane. An electrode is composed of fineparticles of a catalyst metal and a conducting material supporting them,and additionally contains a water repellant as required. The catalystused for the electrode is not particularly limited as long as it is ametal promoting an oxidation reaction of hydrogen and a reductionreaction by oxygen. Catalysis include platinum, gold, silver, palladium,iridium, rhodium, ruthenium, iron, cobalt, nickel, chromium, tungsten,manganese, vanadium and alloys thereof. Of these metals, platinum ismainly used. The conducting material may be any electron-conductivematerial such as various kinds of metals and carbon materials. Thecarbon materials include, for example, carbon black such as furnaceblack, channel black and acetylene black; activated carbon; andgraphite, used alone or in combination. The water repellant ispreferably a fluorine-containing resin having water repellency, and morepreferably one having excellent heat resistance and oxidationresistance. Such materials include, for example,polytetrafluoroethylene, tetrafluoroethylene-perfluoroalkylvinylethercopolymer, and tetrafluoroethylenehexafluoropropylene copolymer. As suchan electrode, for example, an electrode made by E-TEK is widely used.

In order to manufacture MEA from the above-described electrode and anion exchange resin membrane, for example, the following method is used.An ion exchange fluorocarbon resin is dissolved in a mixed solvent ofalcohol and water to prepare a solution, in which carbon-supportingplatinum, as an electrode material, is dispersed to make a paste. Thispaste is then applied onto PTFE sheets in a specified amount and dried.Then, said PTFE sheets are placed so that the coated surfaces face eachother with an ion exchange resin membrane being sandwiched between thecoated surfaces. These are the bonded using a hot press. The temperatureof the hot press depends on the type of ion exchange resin membrane, butusually is 100° C. or above, preferably 130° C. or above, and morepreferably 150° C. or above. Another method of manufacturing MEA isdescribed in “J. Electrochem. Soc., Vol. 139, No. 2, L28–L30 (1992).”According to this method, an ion exchange fluorocarbon resin isdissolved in a mixed solvent of alcohol and water, and a solutionconverting to SO₃Na type is prepared. To this solution,platinum-supporting carbon is added to obtain an ink-like solution. Theink-like solution is applied onto a surface of an ion exchangefluorocarbon resin membrane, which has been converted to SO₃Na type inadvance. The solvent is then removed. Finally, all the ion exchangegroups are converted again to the SO₃H type to obtain an MEA. Thepresent invention can be applied to such an MEA.

(Manufacturing Method of a Fuel Cell)

Next, a method for manufacturing a solid-polyelectrolyte-type fuel cellwill be described. A solid-polyelectrolyte-type fuel cell is composed ofan MEA, current collectors, a fuel-cell frame, a gas-supplying deviceand the like. Among them, the current collector (bipolar plate) is aflange made of graphite or metal, having gas passages at the surface orthe like, which function to transfer electrons to an external loadcircuit, and supply hydrogen or oxygen to the MEA surface. The fuel cellcan be fabricated by inserting the MEA between such current collectorsand piling up a plurality of the laminates. The fuel cell is operated bysupplying hydrogen to one electrode, and oxygen or air to the otherelectrode. A higher operating temperature of the fuel cell is preferablebecause the catalytic activity is enhanced, but the operatingtemperature is usually 50 to 100° C. at which the water content iseasily controlled. On the other hand, a reinforced ion exchange membraneof the present invention may be operated at 100 to 150° C. by improvingstrength at high temperature and in high humidity. Although a higherfeed pressure of oxygen or hydrogen is preferable for increased outputof the fuel cell, the pressure is preferably adjusted within a suitablepressure range to reduce the probability of contact of both materialscaused by breakdown of the membrane or the like.

The present invention will be described in more detail by the followingExamples. Testing methods for the properties shown in the Examples areas follows.

-   (1) Melt Index

The melt index of the precursor of an ion exchange fluorocarbon resinmeasured in accordance to JIS K-7210 at a temperature of 270° C. andunder a load of 2.16 kg was made the MI (g/10 min).

-   (2) Membrane Thickness

An acid-type ion exchange membrane was allowed to stand for 12 hours ormore in a constant-temperature chamber maintained at a temperature of23° C. and a relative humidity of 65%. The thickness was measured with amembrane thickness gauge (made by Toyo Seiki Seisaku-Sho Ltd.: B-1).

-   (3) Equivalent Puncture Strength

An acid-type ion exchange membrane was allowed to stand for 12 hours ormore in a constant-temperature chamber maintained at a temperature of23° C. and a relative humidity of 65%. A puncture test was conductedusing a handy compression tester (KES-G5 made by KATO TECH Co. Ltd.)under test conditions of a radius of curvature of the probe tip of 0.5mm, and a puncture speed of 2 mm/sec. The puncture strength (g) wasdefined as the maximum puncture load. The equivalent puncture strength(g/25 μm) was calculated from the puncture strength multiplied by 25(μm)/membrane thickness (μm).

-   (4) High-temperature Break Strength

An acid-type ion exchange membrane was allowed to stand for 12 hours ormore in a constant-temperature chamber maintained at a temperature of23° C. and a relative humidity of 65%. A sample cut to a membrane widthof 5 mm was placed in a tensile tester (Shimadzu Corporation: AGS-1kNG)set at 220° C. The tensile test was conducted with a distance betweenchucks of 20 mm and a pulling speed of 10%/sec to obtain the breakingstrength E (kg/cm²) at 220° C.

-   (5) Water Content

After an acid-type ion exchange membrane was immersed in purified waterat 25° C. for 30 minutes, water on the membrane surface was wiped off,and the membrane in the water-containing state was weighed at 23° C.Thereafter, the sample was dried at 110° C. for 1 hour or more, and thesample in the dry state was weighed taking care not to absorb moisture.The water content, W (%), was determined from these values using thefollowing equation:W={(Wa−Wb)/Wb}×100where, Wa is the weight in the water-containing state (g) and Wb is theweight in the dry state (g).

-   (6) Storage Modulus (Ion Exchange Fluorocarbon Resin Precursor    Membrane (Having SO₂F as the Ends of the Side Chains))

A sample of a membrane having a width of 5 mm cut from the precursormembrane was placed in a dynamic viscoelesticity meter (IT Measurement &Control Co. Ltd.: DVA-200), and the dynamic viscoelesticity in thetensile mode was measured in accordance with JIS K-7244 with a distancebetween chucks of 20 mm, a strain of 0.1%, a frequency of 35 Hz, and atemperature elevation speed of 5° C./min to determine the storagemodulus E′ (Pa) at a predetermined temperature.

-   (7) Storage Modulus (Ion Exchange Fluorocarbon Resin Membrane    (Having SO₃H the Ends of the Side Chains))

A sample of a membrane width of 5 mm cut from an acid-type ion exchangeresin membrane which was allowed to stand for 12 hours or more in aconstant temperature chamber maintained at a temperature of 23° C. and arelative humidity of 65% was placed in a dynamic viscoelesticity meter(IT Measurement & Control Co. Ltd.: DVA-200), and the dynamicviscoelesticity in the tensile mode was measured in accordance with JISK-7244 with a distance between chucks of 20 mm, a strain of 0.1%, afrequency of 35 Hz, and a temperature elevation speed of 5° C./min todetermine the storage modulus E′ (Pa) at a predetermined temperature.

-   (8) Storage Modulus (Ion Exchange Fluorocarbon Resin Membrane    (Having SO₃NH(Bu)₃ as the Ends of the Side Chains))

An acid-type ion exchange resin membrane was immersed in 50 ml of atributyl amine solution at about 23° C., and allowed to stand for 12hours or more with stirring every 3 to 4 hours. The sample was taken outof the solution, and tributyl amine on the membrane surface was washedwith a large quantity of water. After allowing the sample to remain for12 hours or more in a constant temperature chamber maintained at atemperature of 23° C. and a relative humidity of 65%, the sample of amembrane having a width of 5 mm cut from the precursor membrane was setin a dynamic viscoelesticity meter (IT Measurement & Control Co. Ltd.:DVA-200), and the dynamic viscoelesticity in the tensile mode wasmeasured in accordance with JIS K-7244 with a distance between chucks of20 mm, a strain of 0.1%, a frequency of 35 Hz, and a temperatureelevation speed of 5° C./min to determine the storage modulus E′ (Pa) ata predetermined temperature.

-   (9) Heat of Crystal Fusion at 270 to 350° C.

About 20 mg of an ion exchange fluorocarbon resin precursor membrane(accurately weighed) was placed in a sealed Al sample vessel, and heatedfrom 25° C. to 380° C. at a temperature elevation speed of 20° C./minusing a DSC instrument (Parkin-Elmer: Pyris-1) to obtain a DSC curve.From the difference between this DSC curve and the DSC curve previouslyobtained from the sealed Al sample vessel alone, a differential DSCcurve was again calculated, and the endothermic quantity of the peak ofcrystal fusion between 270° C. and 350° C. in the differential DSCcurve-was calculated.

-   (10) Equivalent Weight

About 0.05 to 0.10 g of an acid-type ion exchange membrane was immersedin 30 ml of a saturated NaCl aqueous solution at 25° C. The solution wasallowed to stand for 30 minutes while stirring, andneutralization-titrated with a 0.01-N sodium hydroxide aqueous solutionusing phenolphthalein as an indicator, to an equivalent point where thevalue indicated by a pH meter (Toko Chemical Laboratories Co., Ltd.:TPX-90) was within a range between 6.95 and 7.05. The Na-type ionexchange resin membrane obtained after the neutralization was rinsedwith purified water, then vacuum dried and weighed. The equivalentweight, EW (g/eq), was obtained from the equivalents of sodium hydroxiderequired for neutralization, M (mmol) and the weight of the Na-type ionexchange resin membrane, W (mg) using the following formula:EW=(W/M)−22

-   (11) Horizontal Ionic Conductivity at 25° C.

An acid-type ion exchange membrane was cut into a strip with a width of1 cm, and immersed in purified water at 25° C. for 1 hour or more. Theimmersed membrane was cut again accurately into a strip with a width of1 cm, and on the surface of the membrane, 6 electrode wires of adiameter of 0.5 mm were contacted in parallel at an interval of 1 cm.After allowing the sample to stand for 12 hours or more in aconstant-temperature chamber adjusted to a temperature of 25° C. and arelative humidity of 98%, the resistance was measured by an A.C.impedance method (10 kHz), and the resistance per unit length wasdetermined from the electrode distance and the resistance. Thehorizontal ionic conductivity at 25° C., Z (S/cm), was calculated usingthis value from the following equation:Z=1/membrane thickness (cm)/membrane width(cm)/resistance per unitlength (Q/cm).

EXAMPLE 1

In a 20 liter stainless steel autoclave, 17.61 kg ofCF₂═CF—O—CF₂CF(CF₃)—O—CF₂CF₂—SO₂F was fed, purged with nitrogen, andthen charged with tetrafluoro ethylene (TFE, CF₂═CF₂). After adjustingthe temperature to 25° C. and the pressure of TFE to 0.645 MPa-G (gaugepressure), 106 g of a monomer solution containing 5 wt % (n-C₃F₇COO—)₂was added to carry out polymerization. The polymerization was carriedout for 30 minutes while intermittently feeding TFE from outside thepolymerization vessel, and lowering the TFE pressure from an initialvalue of 0.645 MPa-G to a final value of 0.643 MPa-G. After purging TFEin the polymerization vessel system with nitrogen and reducing thepresure to atmospheric pressure, a dispersion liquid of the ion exchangefluorocarbon resin precursor containing a monomer of a solid ratio of8.4 wt % as a dispersion medium was obtained. A quantity of methanolequal to three times the volume of the dispersion liquid was added tothe dispersion liquid to precipitate a slurry. It is allowed to stand toremove the supernatant. The supernatant is removed by washing with about0.5 l of a solution of methanol/CFC113=1/2 (volume ratio). The washingwas repeated 3 times. The sample was dried for 16 hours at 110° C. undera reduced pressure to yield 420.8 g of a powder. The equivalent weightand the melt index of the powder (completely solidified ion exchangefluorocarbon resin precursor) were 928 and 0.052, respectively.

The above-described powder was press-formed into a precursor membranewith a thickness of 53.6 μm. The precursor membrane was immersed in ahydrolysis bath (DMSO:KOH:water=5:30:65) heated to 95° C. for 15 minutesto obtain an ion exchange fluorocarbon resin membrane substituted withpotassium ions. The membrane was washed sufficiently with water andimmersed in purified water of a temperature of 90° C. for 2 hours. Then,the membrane was immersed in a 2-N HCl bath heated to 65° C. for 15minutes to obtain an ion exchange fluorocarbon resin membrane havingacid-type ion exchange groups. The membrane was washed sufficiently withwater and dried to obtain a dried film with a thickness of 53.6 μm. Thecharacteristics of the obtained ion exchange fluorocarbon resin membraneare shown in Table 1.

EXAMPLE 2

An ion exchange fluorocarbon resin membrane with a thickness of 53.0 μmwas formed in the same manner as in Example 1 except that the EW was 950and the MI was 20. It was evaluated similarly. The characteristics ofthe obtained ion exchange fluorocarbon resin membrane are shown inTable 1. Since this membrane has a higher EW than the membrane ofExample 1, the equivalent puncture strength at normal temperature washigher than the strength of Example 1, but the storage modulus at 200°C. was lower than the storage modulus of the membrane of Example 1. Thisshows that the physical entangling is weak due to the lower molecularweight of the membrane of Example 2, and that melt flow was beginning.

EXAMPLES 3 TO 25

Ion exchange fluorocarbon resin membranes having various EWs and MIswere obtained in the same manner as in Example 1 except that theimmersing time in the hydrolysis bath was 1 hour and the immersing timein the hydrochloric acid bath was 12 hours or more. The characteristicsof these ion exchange fluorocarbon resin membranes are shown in Tables 2to 4.

EXAMPLES 26 TO 30

Ion exchange fluorocarbon resin membranes having various EWs and MIswere obtained in the same manner as in Example 2 except that theCF₂═CF—O—CF₂CF₂—SO₂F and tetrafluoroethylene were polymerized byemulsion polymerization. The characteristics of these ion exchangefluorocarbon resin membranes are shown in Table 5.

EXAMPLE 31

The ion exchange fluorocarbon resin membranes of Example 7 and Example14 were immersed in an aqueous solution of water/methanol=25/75 (volumeratio), and heated for 8 hours at 160° C. in an autoclave. After coolingand removal, both ion exchange fluorocarbon resin membranes weredissolved, and no residual weight (gel content) was observed. Theseresults show that these membranes were not cross-linked, and exhibited ahigh heat resistance.

EXAMPLE 32

The ion exchange fluorocarbon resin precursor membrane of Example 14 waspress-formed at 270° C. to obtain a precursor membrane of a thickness of116 μm. The precursor membrane was subjected to 2×2 times simultaneousbiaxial stretching at a stretching temperature of 100° C. using abatch-type simultaneous biaxial stretching machine (Toyo Seiki Co.Ltd.). After stretching, a square stainless steel frame (double-sidedadhesive tape was adhered on one side) was attached to the precursormembrane, and the precursor membrane was removed from the simultaneousbiaxial stretching machine maintaining orientation. Furthermore, anotherframe was attached to the opposite side of the membrane, and the totalsystem was firmly fixed using a clamp. The precursor membrane in thisstate was immersed in a hydrolysis bath (DMSO:KOH:water=5:30:65) heatedto 95° C. for 1 hour to obtain an ion exchange fluorocarbon resinmembrane substituted with potassium ions. This was sufficiently washedwith water, and immersed for 15 minutes in a 2-N hydrochloric acid bathheated to 65° C. to obtain an ion exchange fluorocarbon resin membrane.This was sufficiently washed with water, and dried to obtain a driedmembrane with a thickness of 51 μm.

EXAMPLE 33

The powder of an ion exchange fluorocarbon resin precursor was obtainedin the same manner as in Example 26, except that the MI was 0.2 and theEW was 760. The powder was pelletized twice using a 25-mm uniaxialextruder with a barrel temperature of 270° C. and a rotation speed of 11rpm. Then, a T-die was fixed to the same uniaxial extruder, and sheetforming was performed to obtain a precursor membrane of a thickness of110 μm. At this time, when the thinning of the precursor membrane wastried by elevating the speed of take-up device, breakdown of themembrane occurred. The precursor film was stretched and hydrolyzed inthe same manner as in Example 32 to obtain an ion exchange fluorocarbonresin membrane. This was sufficiently washed with water, and dried toobtain a dried membrane of a thickness of 54 μm.

INDUSTRIAL APPLICABILITY

Since the ion exchange fluorocarbon resin membrane of the presentinvention excels in heat resistance, it has excellent characteristics asthe ion exchange fluorocarbon resin membrane for the operation of a fuelcell at a high temperature.

TABLE 1 Example 1 Example 2 EW (g/eq) 928 950 MI (g/10 min) 0.052 20Membrane thickness (μm) 53.6 56 Equivalent puncture strength (g/25 μm)320 300 Water content (%) 30 29 Storage modulus at 150° C. (Pa) 2.7 ×10⁶  2.7 × 10⁶ Storage modulus at 200° C. (Pa) 2.0 × 10⁶ ※1.0 × 10⁶Storage modulus at 220° C. (Pa) 1.8 × 10⁶ Not measurable because ofbreak 200° C./150° C. Storage modulus ratio 0.73 ※0.33 Molecular weightbetween intertwining 3930 ※7860 point at 200° C. High-temperaturebreaking strength at 5.7 1.2 200° C. (kg/cm²) High-temperature breakingstrength at 3.3 0.01 220° C. (kg/cm²) ※The value measured at 195° C. wasused because the sample was broken at below 200° C. The storage modulusat 200° C. is considered to be less than 1.0 × 10⁶.

TABLE 2 Example 3 Example 4 Example 5 Example 6 Example 7 EW g/eq 826812 812 810 820 MI g/10 min 6.5 2.5 0.56 0.1 0.04 Tg ° C. 118 115 120119 121 Membrane thickness μm 95.2 94.5 140.4 185.1 143.3 Equivalentpuncture g/25 μm 260 260 240 210 240 strength Water content % 42.3 43.138.2 39.6 40.3 Ionic conductivity S/cm 0.13 0.13 0.13 0.12 0.12 E′(150°C.):SO₃H Pa 1.8E+06 2.1E+06 2.4E+06 2.3E+06 2.2E+06 E′(200° C.):SO₃H Pa1.3E+04 2.2E+04 1.2E+06 1.7E+06 1.9E+06 E′(220° C.):SO₃H Pa brokenbroken 3.9E+05 1.4E+06 1.7E+06 E′(200° C.)/ — 0.01 0.01 0.50 0.74 0.86E′(150° C.) Me(200° C.):SO₃H — 605000 357500 6550 4630 4140 Tc:TBA ° C.206.4 197.2 202.4 200.3 206.8 E′(Tc):TBA Pa broken 6.9E+04 6.2E+051.1E+06 1.1E+06 E′(Tc-50):TBA Pa 7.6E+05 9.3E+05 1.2E+06 1.4E+06 1.3E+06E′(Tc)/E′(Tc-50) — — 0.07 0.52 0.79 0.85 High-temperature kg/cm² 0.2 0.92.2 2.9 3.4 breaking strength (220° C.) Heat of crystal J/g <0.05 <0.05<0.05 <0.05 <0.05 fusion

TABLE 3 Example Example Example Example Example 8 9 10 11 12 EW g/eq 966951 959 973 980 MI g/10 min 20 6.7 0.82 0.42 0.11 Tg ° C. 123 124 123123 128 Membrane μm 69.6 115.5 141.6 133.2 150.6 thickness Equivalentg/25 μm 300 300 320 320 320 puncture strength Water content % 24.7 26.225 24.1 23.3 Ionic S/cm 0.09 0.09 0.09 0.08 0.08 conductivity E′(150°C.):SO₃H Pa 3.1E+06 2.8E+06 3.0E+06 2.9E+06 4.0E+06 E′(200° C.):SO₃H Pa9.3E+05 5.2E+05 2.0E+06 1.7E+06 2.6E+06 E′(220° C.):SO₃H Pa 3.3E+042.0E+04 1.6E+06 1.4E+06 2.4E+06 E′(200° C.)/ — 0.30 0.19 0.67 0.59 0.65E′(150° C.) Me(200° C.):SO₃H — 8460 15130 3930 4630 3030 Tc:TBA ° C.257.6 258.7 257.6 258.0 263.2 E′(Tc):TBA Pa broken broken 4.8E+057.1E+05 9.3E+05 E′(Tc-50):TBA Pa 7.5E+05 9.0E+05 1.3E+06 1.5E+06 1.6E+06E′(Tc)/ — — — 0.37 0.47 0.58 E′(Tc-50) High-temperature kg/cm² 0.2 0.31.7 2.6 3.7 breaking strength (220° C.) Heat of crystal J/g <0.05 <0.05<0.05 <0.05 <0.05 fusion Example Example Example Example Example Example13 14 15 16 17 18 EW 951 968 981 1003 979 935 MI 0.07 0.05 0.036 0.010.003 0.0003 Tg 128 126 126 126 129 128 Membrane 215.9 195.5 215.8 227.3213 270.6 thickness Equivalent 280 290 290 280 280 250 puncture strengthWater content 26 24.9 22.5 22.9 23.7 26.8 Ionic 0.09 0.08 0.08 0.07 0.090.09 conductivity E′(150° C.):SO₃H 4.1E+06 3.4E+06 3.4E+06 3.7E+063.5E+06 3.3E+06 E′(200° C.):SO₃H 2.7E+06 2.3E+06 2.4E+06 2.5E+06 2.3E+062.4E+06 E′(220° C.):SO₃H 2.5E+06 2.2E+06 2.3E+06 2.4E+06 2.2E+06 2.3E+06E′(200° C.)/ 0.66 0.68 0.71 0.68 0.66 0.73 E′(150° C.) Me(200° C.):SO₃H2910 3420 3280 3150 3420 3280 Tc:TBA 263.0 261.0 261.2 260.6 264.2 262.7E′(Tc):TBA 1.0E+06 1.0E+06 1.4E+06 1.2E+06 1.3E+06 1.2E+06 E′(Tc-50):TBA1.5E+06 1.4E+06 1.9E+06 1.7E+06 1.8E+06 1.6E+06 E′(Tc)/ 0.67 0.71 0.740.71 0.72 0.75 E′(Tc-50) High-temperature 3.9 3.9 4.6 4.7 5.7 5.2breaking strength (220° C.) Heat of crystal <0.05 <0.05 <0.05 <0.05<0.05 <0.05 fusion

TABLE 4 Example 19 Example 20 Example 21 Example 22 Example 23 Example24 Example 25 EW g/eq 1043 1037 1041 1050 1085 1135 1048 MI g/10 min 201 0.3 0.03 0.0007 0.000007 0.12 Tg ° C. 124 120 128 129 130 133 124Membrane thickness μm 61.4 113.6 153.5 198.3 229.8 206.8 184.7Equivalent g/25 μm 330 320 310 320 300 290 310 puncture strength Watercontent % 19.9 19.9 20.2 19.3 19 16.7 19.5 Ionic conductivity S/cm 0.070.08 0.07 0.07 0.09 0.06 0.07 E′(150° C.):SO₃H Pa 3.9E+06 3.9E+064.4E+06 4.1E+06 5.5E+06 7.0E+06 4.6E+06 E′(200° C.):SO₃H Pa 1.3E+062.5E+06 2.5E+06 2.6E+06 3.1E+06 3.2E+06 2.9E+06 E′(220° C.):SO₃H Pa2.9E+04 2.0E+06 2.2E+06 2.4E+06 2.8E+06 2.8E+06 2.6E+06 E′(200° C.)/ —0.33 0.64 0.57 0.63 0.56 0.46 0.63 E′(150° C.) Me(200° C.):SO₃H — 60503150 3150 3030 2540 2460 2710 Tc:TBA ° C. 259.2 254.7 263.4 264.2 264.6267.6 259.0 E′(Tc):TBA Pa 2.9E+04 8.0E+05 1.1E+06 1.7E+06 1.8E+062.0E+06 1.3E+06 E′(Tc-50):TBA Pa 1.0E+06 1.7E+06 1.9E+06 2.3E+06 2.3E+062.3E+06 2.1E+06 E′(Tc)/E′(Tc-50) — 0.03 0.47 0.58 0.74 0.78 0.87 0.62High-temperature kg/cm² 1.4 2.8 4.2 6.9 11.5 20.3 5.7 breaking strength(220° C.) Heat of crystal J/g <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05fusion

TABLE 5 Example 26 Example 27 Example 28 Example 29 Example 30 EW g/eq823 843 947 980 776 MI g/10 min 5.5 0.07 13 1.3 0.1 Tg ° C. 150 157 151153 150 Membrane thickness μm 112.8 289.7 104.6 218.8 118.4 Equivalentpuncture g/25 μm 320 280 300 340 330 strength Water content % 25.7 20.416.9 15 26.9 Ionic conductivity S/cm 0.09 0.08 0.06 0.06 0.09 E′(150°C.):SO₃H Pa 1.8E+07 3.6E+07 3.9E+07 3.9E+07 2.1E+07 E′(200° C.):SO₃H Pa3.1E+06 3.9E+06 4.9E+06 4.3E+06 3.4E+06 E′(220° C.):SO₃H Pa 2.1E+063.2E+06 3.6E+06 2.7E+06 2.9E+06 E′(200° C.)/E′(150° C.) — 0.17 0.11 0.130.11 0.16 Me(200° C.):SO₃H — 2540 2020 1610 1830 2310 Tc:TBA ° C. 237.2253.1 286.2 288.2 216.5 E′(Tc):TBA Pa 6.8E+05 2.3E+06 broken 4.6E+051.7E+06 E′(Tc-50):TBA Pa 2.0E+06 3.0E+06 8.5E+05 2.5E+06 2.9E+06E′(Tc)/E′(Tc-50) — 0.34 0.77 — 0.18 0.59 High-temperature kg/cm² 2.110.9 3.4 9.8 10.1 breaking strength (220° C.) Heat of crystal J/g <0.05<0.05 0.40 0.50 <0.05 fusion

1. A membrane of an ion exchange fluorocarbon resin comprising acopolymer substantially composed of the following repeating units (A)and (B):

where X is a F atom or a perfluoroalkyl group having 1 to 3 carbonatoms, n is an integer of 0 to 3, m is an integer of 1 to 3, Z is H, Cl,F or a perfluoroalkyl group having 1 to 3 carbon atoms and W′ is CO₂H orSO₃H, and having, after substituted with tributyl ammonium ions, astorage modulus (JIS K-7244) at a temperature of not less than Tcrepresented by the following general equations (1-1) to (1-3) of 0.8×10⁶Pa or more, and a heat of crystal fusion at 270 to 350° C. of 1 J/g orless:Tc (° C.)=55+Tg (EW<750)  (1-1)Tc (° C.)=0.444×EW−278+Tg (750≦EW<930)  (1-2)Tc (° C.)=135+Tg (EW≧930)  (1-3) where Tg designates the peaktemperature of loss tangent in the dynamic viscoelasticity measurementof the ion exchange fluorocarbon resin membrane (having SO₃H as ends ofside chains).
 2. A membrane of an ion exchange fluorocarbon resincomprising a copolymer substantially composed of the following repeatingunits (A) and (B):

where X is a F atom or a perfluoroalkyl group having 1 to 3 carbonatoms, n is an integer of 0 to 3, m is an integer of 1 to 3, Z is H, Cl,F or a perfluoroalkyl group having 1 to 3 carbon atoms and W′ is CO₂H orSO₃H, and having a storage modulus (JIS K-7244) of 1.0×10⁶ Pa or more, aheat of crystal fusion at 270 to 350° C. of 1 J/g or less, an equivalentweight of 950 or less, and a Tg of 135° C. or above.
 3. A membrane of anion exchange fluorocarbon resin comprising a copolymer substantiallycomposed of the following repeating units (A) and (B):

where X is a F atom or a perfluoroalkyl group having 1 to 3 carbonatoms, n is an integer of 0 to 3, m is an integer of 1 to 3, Z is H, Cl,F or a perfluoroalkyl group having 1 to 3 carbon atoms and W′ is CO₂H orSO₃H, and having, after substituted with tributyl ammonium ions, astorage modulus (JIS K-7244) at a temperature Tc represented byequations (1-1), (1-2), and (1-3)Tc (° C.)=55+Tg (EW<750)  (1-1)Tc (° C.)=0.444×EW−278+Tg (750≦EW<930)  (1-2)Tc (° C.)=135+Tg (EW≧930)  (1-3) where Tg designates the peaktemperature of loss tangent in the dynamic viscoelasticity measurementof the ion exchange fluorocarbon resin membrane (having SO₃H as ends ofside chains) or greater than 0.8×10⁶ Pa, and the molecular weight of thefluorocarbon resin is 3,000 or less.
 4. A membrane of afluorocarbon-based ion exchange resin wherein the resin: a) comprises acopolymer substantially composed of the following repeating units (A)and (B):

where X is a F atom or a perfluoroalkyl group having 1 to 3 carbonatoms, n is an integer of 0 to 3, m is an integer of 1 to 3, Z is H, Cl,F or a perfluoroalkyl group having 1 to 3 carbon atoms and W′ is CO₂H orSO₃H; b) has a certain equivalent weight (EW); c) has a crystal meltingheat of one J/g per gram or less when measured at a temperature ofbetween about 270° C. and about 350° C.; d) has acid moieties which canbe substituted with tributyl ammonium ions to produce a substitutedresin; and e) wherein the substituted resin has a storage modulus of atleast 0.8×10⁶ Pa measured according to Japanese Industrial StandardK-7244 at any of three temperatures (Tc) according to Equations 1-1,1-2, or 1-3 as follows: (i) when the substituted resin has an equivalentweight less than 750:Tc=55+Tg  (1-1) (ii) when the substituted resin has an equivalent weightbetween 750 and 930:Tc=0.444×EW−278+Tg  (1-2) (iii) when the substituted resin has anequivalent weight greater than 930:Tc=135+Tg  (1-3) wherein Tc is the temperature in degrees Celsius atwhich storage modulus is measured; wherein Tg is the temperature indegrees Celsius of the peak of the loss tangent in the dynamicviscoelasticity measurement of the substituted resin.
 5. The membraneaccording to any one of claims 1 to 3, wherein a ratio of the storagemodulus at 150° C. to the storage modulus at 200° C. is not less than0.4 and not more than 1.0.
 6. The membrane according to any one ofclaims 1 to 3, wherein a ratio of the storage modulus at Tc−50° C. tothe storage modulus at Tc is not less than 0.4 and not more than 1.0. 7.The membrane according to any one of claims 1 to 3, wherein said ionexchange fluorocarbon resin membrane is not cross-linked.
 8. A method ofmanufacturing the membrane according to any one of claims 1–3 and 4,comprising the steps of forming a film of a precursor of the ionexchange fluorocarbon resin comprising a copolymer of a fluorinatedvinyl compound represented by the general formulaCF₂═CF—(OCF₂CFL)_(n)—O—(CF₂)_(m)—W and a fluorinated olefin representedby the general formulaCF₂═CFZ where L is a F atom or a perfluoroalkyl group having 1 to 3carbon atoms, n is an integer of 0 to 3, m is an integer of 1 to 3, Z isH, Cl, F or a perfluoroalkyl group having 1 to 3 carbon atoms and W is afunctional group convertible to CO₂H or SO₃H by hydrolysis, and thenhydrolyzing the same, wherein said precursor of the ion exchangefluorocarbon resin has a heat of crystal fusion at 270 to 350° C. of 1J/g or less, and a melt index of not less than 7×10⁻⁶ and not morethan
 1. 9. The method according to claim 8, wherein said precursor ofthe ion exchange fluorocarbon resin has a melt index (JIS K-7210) of notless than 7×10⁻⁶ and not more than 0.5.
 10. The method according toclaim 8, wherein said precursor of the ion exchange fluorocarbon resinhas a melt index (JIS K-7210) of not less than 7×10⁻⁶ and not more than0.4.
 11. The method according to claim 8, wherein said precursor of theion exchange fluorocarbon resin has a melt index (JIS K-7210) of notless than 7×10⁻⁶ and not more than 0.1.
 12. The method according toclaim 8, wherein stretching is performed in at least one direction afterfilm formation.
 13. The method according to claim 12, wherein stretchingis performed prior to hydrolysis, and the hydrolysis is performed whilemaintaining orientation.
 14. The method according to claim 12, whereinthe temperature for said stretching is 70° C. or above and below 300° C.15. A membrane electrode assembly comprising an ion exchangefluorocarbon resin membrane according to any one of claims 1 to 3 and 4.16. A solid polyelectrolyte type fuel cell comprising an ion exchangefluorocarbon resin membrane according to any one of claims 1 to 3 and 4.17. The membrane according to any one of claims 1 to 3 and 4, whereinthe copolymer is formed by random copolymerization.